Comment Estimate of 660 meters (Score 3, Informative) 53
But let's do the math: even if these particles are moving at almost the speed of light--300,000 km/s--and they live for 2.2 microseconds, they should only be able to travel about 660 meters before decaying away.
Yet I told you these particles are created at the top of the atmosphere, which is some 100 kilometers, or 100,000 meters up! From our perspective, that muon should never make it to the ground. And yet, it's Einstein to the rescue, thank to the fact that when objects move close to the speed of light, their clocks run slow.